A spurious tone is a tone at the output that is not deliberately created, transmitted, or otherwise intentionally created. Such spurious tones commonly arise in circuits that include multiple unmodulated frequency sources, such as circuits that include local oscillators and/or clocking signals. Example causes of spurious tones may include, for instance, harmonic tones of aggressor sources as well as frequency conversion and intermodulation products. Depending on the application, such spurious tones can be highly undesirable.
Communications applications can be particularly sensitive to spurious signals. For instance, undesirable spurious tones that mix into the final intermediate frequency (IF) of an RF converter tend to manifest in the mixer output and cause receiver performance problems (e.g., down-conversion errors, effective disablement of receiver front-end, communication errors, etc). Such receiver performance problems can be even more significant when the RF converter and the spurious signal source are implemented on the same integrated circuit or substrate.
One way to remedy such spurious issues is to place the RF converter on a separate chip from the spurious source to eliminate on-chip leakage of the spurious source into the converter circuit and/or placing bulky Faraday cages around the converter. However, using separate chips tends to increase the size, weight, and power of system, and places an effective limit on future integration. Another remedy is to back-off power of the local oscillator driver (or other aggressor frequency source). Reducing the aggressor source drive power, however, can have a performance impact on performance parameters such as gain, noise figure, and linearity.
There is a need, therefore, for techniques for eliminating or otherwise sufficiently reducing spurious tones, in applications such as those that employ one or more frequency sources along with a frequency conversion or mixing function, and especially applications implemented as a system-on-chip.